The invention relates to a composition for removing noxious substances, particularly volatile heavy metals and/or organic pollutants, from gases and exhaust gases, and a process for making the composition.
In thermal processes, e.g., metallurgical processes, coal firing and waste incineration, exhaust gases are produced which contain large amounts of volatile heavy metals, such as cadmium, mercury, thallium, arsenic, antimony and lead. These substances are elemental heavy metals or salts thereof, preferably chlorides and oxides.
Usually, the gases are cooled, and the heavy metals are washed out in a wet process. In those cases where the heavy metals are present in elemental form, this process is often not effective. Mercury, for instance, is widely emitted in elemental form. It can not be removed from the gas in a wet process. Add to this the problem that volatile heavy metals are still emitted as aerosols and/or difficulties associated with the separation of fine powder after the wet process.
Another method consists of passing exhaust gas over activated charcoal filters. This process leads to a reduction in the content of volatile heavy metals, e.g., mercury. However, this process is also very expensive because it requires large amounts of highly active carbon and it involves the risk associated with smoldering fires in the bed of coal. Moreover, due to constant abrasion, fine dust containing heavy metals is emitted, which is difficult to separate.
Various attempts have been made in the past to transform the volatile heavy metals into sulfides which are not easily volatilized through the use of sulfur or sulfides, and to remove them from the gas stream.
In this process, sulfur is applied onto the activated charcoal and the gas stream is passed over it. The mercury is quite effectively, removed but the problems regarding self-ignition and the emission of fines remain.
Another state-of-the-art process utilizes suspensions of sulfur or sulfur compounds or aqueous solutions of thionic compounds, e.g., thiocarbamide. The solutions or suspensions are blown into the gas stream, and the resultant compounds of mercury and sulfur are separated out using electrostatic filters.
However, the 70% separation of mercury associated with such processes is too low to guarantee the limit of 0.200 mg/m.sup.3. This insufficient Hg removal is probably due, inter alia, to the addition of water to the gas stream and the considerable content of strong acids in the gas stream when exhaust gases from refuse incinerating plants are treated.
Another alternative which has been proposed in the past involves blowing sulfur or water-insoluble sulfur compounds, e.g., ZnS, together with Ca(OH).sub.2 powder into the exhaust gas stream from refuse incinerating plants and to separate mercury therefrom. Although 1 g/m.sup.3 ZnS at a temperature of from 200.degree. to 250.degree. C. was used, the separation of Hg was only 60% which is too low to ensure an Hg limit of 0.2 mg/m.sup.3.
Consequently, there is the need to specifically separate, perhaps without cooling steps, volatile heavy metals, particularly mercury, in their various compounds and also as elements, as well as organic pollutants such as dioxines and furanes, from gas streams particularly those streams from refuse incinerating plants.
Another object is to develop a simple and inexpensive process for jointly separating out volatile pollutants and acidic constituents from gas streams. For this purpose, the reduction in the content of noxious substances in the exhaust gas stream has to be so great that the existing limits, e.g., for Hg, Cd, Tl and As, can be safely observed.